The present invention relates generally to the recovery of subterranean deposits, and more particularly to a method and system for accessing subterranean deposits from the surface.
Subterranean deposits of coal contain substantial quantities of entrained methane gas limited in production in use of methane gas from coal deposits has occurred for many years. Substantial obstacles, however, have frustrated more extensive development and use of methane gas deposits in coal seams. The foremost problem in producing methane gas from coal seams is that while coal seams may extend over large areas of up to several thousand acres, the coal seams are fairly shallow in depth, varying from a few inches to several meters. Thus, while the coal seams are often relatively near the surface, vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius around the coal deposits. Further, coal deposits are not amendable to pressure fracturing and other methods often used for increasing methane gas production from rock formations. As a result, once the gas easily drained from a vertical well bore in a coal seam is produced, further production is limited in volume. Additionally, coal seams are often associated with subterranean water, which must be drained from the coal seam in order to produce the methane.
Horizontal drilling patterns have been tried in order to extend the amount of coal seams exposed to a drill bore for gas extraction. Such horizontal drilling techniques, however, require the use of a radiused well bore which presents difficulties in removing the entrained water from the coal seam. The most efficient method for pumping water from a subterranean well, a sucker rod pump, does not work well in horizontal or radiused bores.
A further problem for surface production of gas from coal seams is the difficulty presented by under balanced drilling conditions caused by the porousness of the coal seam. During both vertical and horizontal surface drilling operations, drilling fluid is used to remove cuttings from the well bore to the surface. The drilling fluid exerts a hydrostatic pressure on the formation which, if it exceeds the hydrostatic pressure of the formation, can result in a loss of drilling fluid into the formation. This results in entrainment of drilling finds in the formation, which tends to plug the pores, cracks, and fractures that are needed to produce the gas.
As a result of these difficulties in surface production of methane gas from coal deposits, the methane gas which must be removed from a coal seam prior to mining, has been removed from coal seams through the use of subterranean methods. While the use of subterranean methods allows water to be easily removed from a coal seam and eliminates under balanced drilling conditions, they can only access a limited amount of the coal seams exposed by current mining operations. Where longwall mining is practiced, for example, underground drilling rigs are used to drill horizontal holes from a panel currently being mined into an adjacent panel that will later be mined. The limitations of underground rigs limits the reach of such horizontal holes and thus the area that can be effectively drained. In addition, the degasification of a next panel during mining of a current panel limits the time for degasification. As a result, many horizontal bores must be drilled to remove the gas in a limited period of time. Furthermore, in conditions of high gas content or migration of gas through a coal seam, mining may need to be halted or delayed until a next panel can be adequately degasified. These production delays add to the expense associated with degasifying a coal seam.
The present invention provides an improved method and system for accessing subterranean deposits from the surface that substantially eliminates or reduces the disadvantages and problems associated with previous systems and methods. In particular, the present invention provides an articulated well with a drainage pattern that intersects a horizontal cavity well. The drainage patterns provide access to a large subterranean area from the surface while the vertical cavity well allows entrained water, hydrocarbons, and other deposits to be efficiently removed and/or produced.
In accordance with one embodiment of the present invention, a method for accessing a subterranean zone from the surface includes drilling a substantially vertical well bore from the surface to the subterranean zone. An articulated well bore is drilled from the surface to the subterranean zone. The articulated well bore is horizontally offset from the substantially vertical well bore at the surface and intersects the substantially vertical well bore at a junction proximate to the subterranean zone. A substantially horizontal drainage pattern is drilled through the articulated well bore from the junction into the subterranean zone.
In accordance with another aspect of the present invention, the substantially horizontal drainage pattern may comprise a pinnate pattern including a substantially horizontal diagonal well bore extending from the substantially vertical well bore that defines a first end of an area covered by the drainage pattern to a distant end of the area. A first of substantially horizontal lateral well bores extend in space relation to each other from the diagonal well bore to the periphery of the area on a first side of the diagonal well bore. A second set of substantially horizontal lateral well bores extend in space relation to each other from the diagonal well bore to the periphery of the area on a second, opposite side of the diagonal.
In accordance with still another aspect of the present invention, a method for preparing a subterranean zone for mining uses the substantially vertical and articulated well bores and the drainage pattern. Water is drained from the subterranean zone through the drainage pattern to the junction of the substantially vertical well bore. Water is pumped from the junction to the surface through the substantially vertical well bore. Gas is produced from the subterranean zone through at least one of the substantially vertical and articulated well bores. After degasification has been completed, the subterranean zone may be further prepared by pumping water and other additives into the zone through the drainage pattern.
In accordance with yet another aspect of the present invention, a pump positioning device is provided to accurately position a downhole pump in a cavity of a well bore.
Technical advantages of the present invention include providing an improved method and system for accessing subterranean deposits from the surface. In particular, a horizontal drainage pattern is drilled in a target zone from an articulated surface well to provide access to the zone from the surface. The drainage pattern intersected by a vertical cavity well from which entrained water, hydrocarbons, and other fluids drained from the zone can be efficiently removed and/or produced by a rod pumping unit. As a result, gas, oil, and other fluids can be efficiently produced at the surface from a low pressure or low porosity formation.
Another technical advantage of the present invention includes providing an improved method and system for drilling into low-pressure reservoirs. In particular, a downhole pump or gas lift is used to lighten hydrostatic pressure exerted by drilling fluids used to remove cuttings during drilling operations. As a result, reservoirs may be drilled at ultra-low pressures without loss of drilling fluids into the formation and plugging of the formation.
Yet another technical advantage of the present invention includes providing an improved horizontal drainage pattern for accessing a subterranean zone. In particular, a pinnate structure with a main diagonal and opposed laterals is used to maximize access to a subterranean zone from a single vertical well bore. Length of the laterals is maximized proximate to the vertical well bore and decreased toward the end of the main diagonal to provide uniform access to a quadrilateral or other grid area. This allows the drainage pattern to be aligned with longwall panels and other subsurface structures for degasification of a mine coal seam or other deposit.
Still another technical advantage of the present invention includes providing an improved method and system for preparing a coal seam or other subterranean deposit for mining. In particular, surface wells are used to degasify a coal seam ahead of mining operations. This reduces underground equipment and activities and increases the time provided to degasify the seam which minimizes shutdowns due to high gas content. In addition, water and additives may be pumped into the degasified coal seam prior to mining operations to minimize dust and other hazardous conditions, to improve efficiency of the mining process, and to improve the quality of the coal product.
Still another technical advantage of the present invention includes providing an improved method and system for producing methane gas from a mined coal seam. In particular, well bores used to initially degasify a coal seam prior to mining operations may be reused to collect gob gas from the seam after mining operation. As a result, costs associated with the collection of gob gas are minimized to facilitate or make feasible the collection of gob gas from previously mined seams.
Still another technical advantage of the present invention includes providing a positioning device for automatically positioning down-hole pumps and other equipment in a cavity. In particular, a rotatable cavity positioning device is configured to retract for transport in a well bore and to extend within a down-hole cavity to optimally position the equipment within the cavity. This allows down-hole equipment to be easily positioned and secured within the cavity.